1. Field of the Invention
The present invention relates to audio amplifier circuits and, more particularly, to audio amplifier circuits employing power factor correction circuitry to improve the current draw characteristics from an AC line.
2. Related Art
With reference to FIGS. 1 and 2, two conventional audio amplifiers are shown. The audio amplifier of FIG. 1 includes terminals for connection to an AC line 12, a linear amplifier 10, and a power amplifier 20. The power amplifier 20 is connected to a speaker 22 as is known in the art.
The linear power supply 10 includes an AC line transformer 14, for example a 60 Hz, 120 V step down transformer, a rectifier 16 (for example, a full bridge rectifier) and a filter capacitor 18. The linear power supply operates to convert the AC power from the AC line 12 to DC power across filter capacitor 18 for connection to the power amplifier 20. The power amplifier 20 converts the low power signal input to the power amplifier 20 to a high power signal for delivery to the speaker 22.
The audio amplifier of FIG. 1 suffers from several disadvantages. In particular, the AC line transformer 14 is typically very large and heavy due to the relatively low frequency of the AC line. Further, the filter capacitor 18 must have a relatively high value of capacitance which may require multiple capacitors, again because of the low frequency of the AC line 12.
Moreover, the input current drawn by the linear power supply 10 from the AC line 12 is not ideal. Indeed, with reference to FIG. 5 it is noted that the current I flowing from the AC line 12 into the linear power supply 10 has a relatively high peak current as compared to the voltage waveform of the AC line. Indeed, the current I does not substantially correspond to the wave shape of the voltage on the AC line 12 and, therefore, creates distortion on the AC line 12. This distortion is very undesirable because it creates harmonics on the AC line 12.
Further, the audio amplifier of FIG. 1 suffers from the additional disadvantage that the maximum power deliverable from the linear power supply 10 to the power amplifier 20 reduces as the voltage on the AC line 12 reduces. As may be seen in FIG. 3, as the AC line voltage reduces from 120 volts to 65 volts the loss percentage of the power deliverable from the linear power supply 10 moves from the ideal value of 1.00 towards about 0.30. As a result, the maximum power delivered to the speaker 22 is a function of the voltage on the AC line 12. This is severely problematic when the audio amplifier is connected to AC lines 12 having lower voltage levels or varying levels.
The audio amplifier of FIG. 2 includes terminals for connection to the AC line 12, a switching power supply 11, and a power amplifier 20. The power amplifier 20 is coupled to a speaker 22 as is known in the art. The switching power supply 11 includes an input rectifier 13 coupled to the AC line 12, a filter capacitor 17, a switch 24, a high frequency transformer 15, a rectifier 16, and a filter capacitor 18'. The input rectifier 13, for example, a full bridge rectifier, in combination with the filter capacitor 17 produces a rough source of DC power from the AC line 12. The switch 24, which may be implemented using a semiconductor transistor or the like, is coupled in series with the primary of the high frequency transformer 15 so that a voltage is produced on the secondary winding of the transformer which has desirable characteristics.
Typically, there is a closed loop control system which senses the voltage on the secondary of the high frequency transformer 15 and uses the sensed voltage to control the switch. 24 More particularly, the switch 24 is controlled to produce a pulse width modulated signal on the secondary winding of the transformer 15. As is known in the art, the switch 24 is turned on and off at a relatively high frequency (for example, 70 KHz) as compared to the frequency of the voltage on the AC line 12 such that very efficient conversion of the DC voltage level across filter capacitor 17 to a lower DC voltage across filter capacitor 18' is obtained. Rectifier 16 in combination with filter capacitor 18' produce a smooth DC voltage level for input to the power amplifier 20. The power amplifier 20 converts the low power signal input thereto to a high power signal for delivery to the speaker 22.
The audio amplifier of FIG. 2 overcomes some of disadvantages of the circuit shown in FIG. 1. Namely, the circuit of FIG. 2 requires a relatively smaller transformer 15 as compared to the AC line transformer 14 of FIG. 1 because the frequency of operation of the transformer 15 is much higher than the frequency of the AC line 12. Further, capacitors 17 and 18' may be much smaller in capacitance than capacitor 18 of the linear power supply 10, again because the frequency of operation of the switch 24 and transformer 15 are much higher than the frequency of the AC line 12.
The audio amplifier of FIG. 2, however, still suffers from several disadvantages. Namely, as was the case with the circuit of FIG. 1, the power supply 11 draws current I from the AC line 12 having relatively high peak values as is shown in FIG. 5. In addition, the maximum power deliverable from the switching power supply 11 to the power amplifier 20 drops as a function of the voltage level of the AC line 12. Consequently, when the circuit of FIG. 2 is utilized in a country having a standard AC line 12 of, for example, 100 volts, the maximum power deliverable from the power amplifier 20 to the speaker 22 drops substantially (see FIG. 3).
Accordingly, there is a need in the art for an audio amplifier system which is capable of drawing current from the AC line which closely follows the wave shape of the voltage on the AC line and is capable of delivering a relatively constant maximum power level to the speaker 22 which is in variable as a function of either the frequency or the voltage level of the AC line 12.